Grain size
Particle size, also called grain size, means the diameter of individual grains of sediment, or the lithified particles in clastic rocks. The term may also be used for other granular materials.[1]
| φ scale | Size range (metric) |
Size range (approx. inches) |
Aggregate name (Wentworth Class) |
Other names |
|---|---|---|---|---|
| < −8 | > 256 mm | > 10.1 in | Boulder | |
| −6 to −8 | 64–256 mm | 2.5–10.1 in | Cobble | |
| −5 to −6 | 32–64 mm | 1.26–2.5 in | Very coarse gravel | Pebble |
| −4 to −5 | 16–32 mm | 0.63–1.26 in | Coarse gravel | Pebble |
| −3 to −4 | 8–16 mm | 0.31–0.63 in | Medium gravel | Pebble |
| −2 to −3 | 4–8 mm | 0.157–0.31 in | Fine gravel | Pebble |
| −1 to −2 | 2–4 mm | 0.079–0.157 in | Very fine gravel | Granule |
| 0 to −1 | 1–2 mm | 0.039–0.079 in | Very coarse sand | |
| 1 to 0 | 0.5–1 mm | 0.020–0.039 in | Coarse sand | |
| 2 to 1 | 0.25–0.5 mm | 0.010–0.020 in | Medium sand | |
| 3 to 2 | 125–250 µm | 0.0049–0.010 in | Fine sand | |
| 4 to 3 | 62.5–125 µm | 0.0025–0.0049 in | Very fine sand | |
| 8 to 4 | 3.90625–62.5 µm | 0.00015–0.0025 in | Silt | Mud |
| > 8 | < 3.90625 µm | < 0.00015 in | Clay | Mud |
| >10 | < 1 µm | < 0.000039 in | Colloid | Mud |
In some schemes "gravel" is anything larger than sand (>2.0 mm), and includes "granule", "pebble", "cobble", and "boulder" in the above table. In this scheme, "pebble" covers the size range 4 to 64 mm (−2 to −6 φ).
Grain Size Media
Wentworth grain size chart from United States Geological Survey Open-File Report 2006-1195: Note size typos; 33.1mm is 38.1 & .545mm is .594
Beach cobbles at Nash Point, South Wales
References
- ↑ Size ranges define limits of classes that are given names in the Wentworth scale (or Udden-Wentworth) used in the United States. The Krumbein phi (φ) scale, a modification of the Wentworth scale created by W. C. Krumbein, (Krumbein & Sloss 1963) is a logarithmic scale computed by the equation:
- [math]\displaystyle{ \phi=-\log_2{D/Do}\, }[/math]
- [math]\displaystyle{ \phi }[/math] is the Krumbein phi scale, and
- [math]\displaystyle{ D }[/math] is the diameter of the particle
- [math]\displaystyle{ Do }[/math] is a reference diameter, equal to 1 mm (to make the equation dimensionally consistent.)
- [math]\displaystyle{ D=Do \times 2^{-\phi}\, }[/math]
- W C Krumbein & L L Sloss, Stratigraphy and Sedimentation, 2nd edition (Freeman, San Francisco, 1963).
- J A Udden, Mechanical composition of clastic sediments, Bull. Geol. Soc. Am. 25, 655-744 (1914).
- C K Wentworth, A scale of grade and class terms for clastic sediments, J. Geology V. 30, 377-392 (1922).